Major exchanges of the indirect-control type, using crossbar or cross-point switching, are generally provided with malfunction detectors which can be summoned by circuit components such as registers, markers or senders, for example, to check on their operation whenever any of these components fails to complete a prescribed sequence of steps or otherwise departs from its normal performance. The fault giving rise to the intervention of the malfunction detector does not necessarily lie within the summoning component itself but may be due to a defect elsewhere in the associated circuit. Thus, it may be necessary to trace the source of the malfunction by correlating error signals from several coacting circuit elements and also taking into account the nature of these error signals which may be suitably coded or timed to indicate various types of defects, such as an unsuccessful parity check or a deviation from a normal voltage level. Heretofore, this tracing of defective circuit components has been largely performed by human operators examining the error codes emitted by a malfunction detector, usually in the form of a printout. Such mental correlation of failure signals is time-consuming and not always reliable since it depends on the skill of the operator. Prior attempts at automating the defect-tracing procedure have not been commercially successful, mostly on account of the complexity and high cost of the processing equipment used for this purpose.